Raindrops keep falling on my flowerhead

Imagine you’re a young and eager pollen. You’re eager for a hot date with an ovule, but first you have to get to her doorstep. How do you travel? The popular way is to fly and many pollen grains are light enough to get lifted on the breeze as the nostrils of the world’s hayfever sufferers know well. Heavier pollen hitches a lift from an insect or, in more stylish cases, a bird. Another route is through the water. Zostera marina is a marine grass that is rarely exposed above the water surface, so it’s no surprise it’s evolved submarine pollination. A Chinese-Canadian team has found another aquatic route, but this in a plant you might not expect. It’s one of Charles Darwin’s favourite land-based plants, an orchid.

Acampe rigida comes from south-west China, but it would look great in my garden because A. rigida does something very peculiar. It flowers when it rains.

Acampe rigida is ready for the rain. Photo by Badlydrawnboy22 / Wikimedia Commons.

The flowering season coincides with the rainy season in south-west China. This is odd time to aim to flower. Normally plants like to flower when insects are around, and these aren’t so common when it rains. What’s going on?
Something that could be happening is that the orchids are using a weird insect for pollination. So the team set up to watch the orchids as they flowered and see who visited. They did this in 2009 and 2010. The result was in 2009 there was one visitor to flower, a wasp. In 2010 there was nothing. Whatever was pollinating the flowers, it wasn’t insects, so what else is around during the rainy season?

Raindrops.

The team asked if the raindrops were pollinating the flowers and set decided to run a test. What they did was simple and elegant. They put a shelter over some orchids and compared the pollination of the sheltered and exposed flowers. There are some problems with this. A simple umbrella won’t work because it’ll shade the plant from sunlight. You also need to allow for wind transfer. Despite this they set up transparent shelters and found that the orchids hit by the rain were much more successful with pollination.

More experiments with a shower head to simulate rain revealed what was happening.

Raindrops hit the anther cap, knocking it out of the way and exposing the pollen. They’re bounced up and then pulled back down on to the stigmas. Contact, and self-pollination happens. Effectively the raindrops are twanging the pollen onto the stigmas. The team have a video of this happening on their blog.

This is strange stuff. In their paper Rain pollination provides reproductive assurance in a deceptive orchid Fan et al. note that most research shows that you don’t want reproductive organs exposed to rain. It’s potential damage to equipment that you need working to entice insects. Nonetheless Fan et al. have a mechanism and the results to show that rain has a positive effect.

The paper is also a demonstration of the long reach of research. The oldest citation in the paper is Sprengel’s work on pollination from 1793. Darwin also gets name checked. It’s perfectly possible that Fan, Barrett, Lin, Chen, Zhou and Gao will be name checked in a couple of centuries’ time in further research on aquatic pollination. It puts the Impact Factor into perspective.

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About the author

Alun Salt

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?